The challenges are cheating. During and leading up to christmas and maybe day after or two, challenges became super easy, I even managed to win 4 keys in a day, despite extra time added to each malas.
But now it's tricky, your first try with zero malas is super easy, the amount of time left after completing your first challenge of the day makes you think of that christmas time and you can win 3 or 4 keys. But as you win, it's ot only the malas that increases, the difficulty of the challenges also increases. I had malas discount for giving away a game and took off 4 malas for mining leaving only 8 second malas. Mining is my best challenge, but I wasn't able to win despite having mostly played it flawlessly. And it wasn't even close.
Also tricky is, other halmarks of easier are still present, like more of the weaker brown rocks and more of the light brown twigs than exactly 40. But as you win, the "difficulty" increase seems to be in the form of just moving and acting slower.
It seems they have taken out some of the personal skill in these challenge games, making it so that a first key is pretty much assured and a third key is pretty much impossible no matter your skill. This isn't a criticism per-say, it makes sense to level the playing field so everyone gets 1-2 keys and no more. But it does mean that after you get 2 keys during the day, don't waste the rest of your silver on a third key.(I know as I say this this means more competition in the game lottery, but meh, also not everyone reads here)